This invention relates to a semiconductor array having a semiconductor layer and a layer of a ferromagnetic material coupled to it, wherein the layer of ferromagnic material injects a spin-polarized state of semiconducting charge carriers into the semiconductor layer.
The goal of creating such an arrangement is to produce in a semiconducting material, e.g., GaAs or Si, which can be used to produce semiconductor components a spin state having the highest possible polarization, so that not only the charge carriers as such but also the spin of the charge carriers in the semiconductor component serve as an information medium. In this way, it is possible to achieve much shorter switching times, and, as a result, to produce faster semiconducting components (e.g., so-called spin transistors).
One problem with the spin electronics (spintronics) described above is to achieve a sufficiently high spin polarization in a conventional semiconducting material such as GaAs or Si.
It has been proposed to apply a ferromagnetic metal to a semiconducting layer. A current flowing through such a ferromagnetic material is always spin polarized, since the magnetism in metallic ferromagnetic materials is produced by spin polarization of the charge carriers. However, in the past, these spin-polarized charge carriers could not be transmitted into the adjacent semiconductor to a sufficient extent. This is because, as a result of the difference in the resistance of metal, on the one hand, and semiconductors on the other hand, injection of spin polarization from a metal into a semiconductor is very inefficient. This can be attributed to the fact that the total electric resistance of a metal-semiconductor component is determined by the resistance of the semiconductor, which in turn is independent of the direction of spin.
The invention aims at creating a semiconductor array of the above type having optimized spin polarization of the semiconductor charge carriers in the semiconducting layer.
This problem is solved in accordance with the present invention by creating a semiconductor array having the features of claim 1.
Accordingly, a first semiconductor layer (preferably formed from GaAs or Si) is coupled to a second semiconducting layer of a ferromagnetic material (e.g., EuS), so that a spin-polarized state of the semiconducting charge carriers is injected into the first semiconducting layer, and the second semiconducting layer interacts with charge carriers outside of the second semiconducting layer (which are, e.g., made available through an additional metal layer), to create a highly spinpolarized state of the second semiconducting layer at a temperature of at least 250 Kelvin, preferably at at least 270 Kelvin and especially preferably at at least 300 Kelvin.
The present invention is based on the surprising finding that, at temperatures above room temperature, a high spin polarization can exist in a ferromagnetic semiconductor such as EuS if said ferromagnetic semiconductor interacts with additional (free) charge carriers.
Such a spin polarized semiconductor, in which the spin-polarized (magnetic) state exists at temperatures of 250 Kelvin and above, is in turn suitable for injecting spin polarization into a first semiconducting layer, which is formed by a traditional semiconductor (not magnetic) which is especially suitable for rapid switching operations, e.g., GaAs, Si or by some other elemental semiconductor and/or conventional III-V or II-VI semiconductors. This is because the semiconducting properties of the ferromagnetic material (second semiconducting layer) ensure that the electric resistance of said second semiconducting layer is comparable to the electric resistance of the first semiconducting layer (made of GaAs and/or Si) so that the high spin polarization of the second semiconducting layer can be transmitted into the first semiconducting layer.
However, the fact that the known ferromagnetic semiconductors have Curie temperatures far below 250 Kelvin is in conflict with the solution according to this invention. Among the Eu chalcogenides, for example, the material EuO has the highest Curie temperature of approximately 110 Kelvin. The prerequisite for achieving the goal according to this invention was therefore the discovery that the Curie temperatures in ferromagnetic semiconductors can be increased up to temperatures of above 160 K and that above 250 Kelvin a high spinpolarization is created by bringing this semiconducting ferromagnetic material into interaction with other charge carriers.
The ferromagnetic second semiconducting layer may be formed by a compound formed from a rare earth element and a respective crystal-forming substance. The rare earths, e.g., Eu and Gd, are particularly well suited because they include elements having a half-filled f shell which therefore have, at low temperatures, a completely spin-polarized ground state. The chalcogens, e.g., O, S or Se (elements of group VI of the periodic system of elements) are examples of suitable respective crystal-forming substances.
The respective crystal-forming substance and, if necessary, the stoichiometry are selected in a way that the element of the rare earths responsible for the spin-polarized state has exactly the valence that results in a magnetic ground state having the highest possible spin polarization. In the case of Eu, this is a positive charge of 2. Thus, chalcogens, e.g., O and S, are suitable as crystal-forming elements. In the case of Gd, a positive charge of 3 is the goal. Accordingly, a suitable crystal-forming substance would include, e.g., N. When using chalcogens with Gd, a stoichiometry of Gd2X3 (X=chalcogen) will be required.
Suitable materials for the second semiconducting layer include, but are not limited to, EuS, EuO, EuSe, SmS, Gd2Se3, Gd2S3 or YbS. These semiconducting compounds are characterized by a spin-polarized ferromagnetic state at low temperatures. To maintain a spin -polarized state at even higher temperatures, preferably above 250 Kelvin or even above 300 Kelvin, a metallic layer may be placed adjacent to the second semiconducting layer. This metallic layer can be formed in particular by a ferromagnetic transition metal, e.g., Co, Fe, Ni or an alloy of such elements.
If the second semiconducting layer is formed by a ferromagnetic semiconductor based on a rare earth element, e.g., EuS, then the d-electrons of a transition metal adjacent to it will generate a charge carrier population in the essentially empty d-band of the second semiconducting layer. This increases the magnetic coupling and thus creates a spinpolarization in the ferromagnetic semiconductor.
Thus, the ground state of the 4f-electrons responsible for the magnetism in EuS is a pure spin state with a half-full 4f-shell, and the d-type conduction band is strongly spin polarized below the Curie temperature with an exchange splitting of 0.18 eV. The energy of this splitting is thus far above the thermal energy of approximately 30 meV at 300 Kelvin and is thus also relevant at room temperature. As a result, electrons in the d-type conduction band are completely spin-polarized, thus permitting an increase in the spinpolarization of EuS through interaction with the d-electrons of a ferromagnetic transition metal.
Accordingly, a ferromagnetic semiconductor having a resistance similar to that of GaAs or Si and also having a spin-polarized (ferromagnetic) state at low temperatures can be created by combining a rare earth element with the respective crystal-forming substance. By interaction with a transition metal layer, the spinpolarization of such a ferromagnetic semiconductor can be influenced to such a degree, that the spinpolarization is even present at room temperature, i.e., at temperatures of 300 Kelvin and even at higher temperatures. The ferromagnetic semiconductor is therefore suitable for producing a spin-polarized state in, e.g., GaAs or Si. For this purpose, the GaAs or Si semiconductor is coupled to the ferromagnetic semiconductor described above, e.g., by designing the two semiconductors as adjacent layers.
In a preferred embodiment of this invention and to achieve particularly high spin polarization in the ferromagnetic semiconducting layer, this semiconducting layer and the metal layer are components of a layer structure in which layers of the semiconducting ferromagnetic layer type and layers of the transition metal layer type are arranged alternately one above the other. A periodic layer structure is particularly suitable.
In a preferred embodiment, the thickness of the semiconducting layers of the ferromagnetic type is at least great enough to produce a spinpolarization, but is not greater than the exchange length of the spin-polarized electrons with the adjacent metal layer.
In one, but not all, embodiments of the present invention, the thickness of the (transition) metal layers is at least great enough so that the Curie temperature is greater than 250 K in the case of a ferromagnetic layer (i.e., at least two atomic layers in the case of Co) and that, in the case of a nonmagnetic layer, a closed layer is formed.
As an alternative to coupling the ferromagnetic semiconducting layer with a metallic layer, the Curie temperature of the ferromagnetic semiconducting layer may also be raised by suitably doping an adjacent first semiconducting layer (GaAs or Si layer).
In a preferred embodiment, the second ferromagnetic semiconducting layer is directly adjacent to the first semiconducting layer (formed by GaAs and/or Si). In another preferred embodiment, an intermediate layer may be provided between these two semiconducting layers.
In an even more preferred embodiment of this invention, the second ferromagnetic semiconducting layer is formed by EuS, and the metal layer used to raise the spinpolarization is formed by Co. In such an embodiment, these two layers are joined by antiferromagnetic coupling, which can result, at temperatures above 250 Kelvin, in high spinpolarization.
According to another aspect of this invention, a semiconducting layer of a ferromagnetic material which, as discussed above, interacts with charge carriers made available outside of the semiconducting layer, such that, above 250 Kelvin, a high spin polarization is created, is used as the source for spin-polarized electrons and/or as a spin filter for electron detection, which can, e.g., be used in electron microscopy.